1. Field of the Invention
The present generally relates to techniques for wireless communication, more particularly, to a TV-band white space transmitter (TX) capable of managing the unwanted harmonic emission and rejecting substantially more harmonics than conventional arrangements.
2. Description of the Related Art
Improvements of sharing the geographically unused white spaces in TV band (54 to 864 MHz) has led to the development of IEEE 802.22 wireless regional area network (WRAN) [1], which embodies the cognitive radio (CR) techniques to enable an opportunistic share of the spectrum [2]. To avoid causing detrimental interference to the incumbent primary users, one key challenge of such a TV-band transmitter (TX) lies on managing the unwanted harmonic emission [3] without pricy or complicated filtering modules [4].
Harmonic rejection mixers (HRMs), which are designed to reject local oscillator (LO) harmonics (or “images”), permits a saving on radio frequency (RF) band-filtering to be made, for example in software-defined radio (SDR). Usual implementations of HR mixers use a weighted combination of hard-switching mixers that need careful alignment of phase and gain parameters. Hard-switching mixer is one of the sources generating influential sidebands associated with the harmonics of the local oscillator (LO). Multi-path mixers feature the desired integratability and wideband ability in harmonic rejection [5]. A typical differential mixer already can stem all even harmonics, while rejecting the odd ones entails additional paths. The 6-path harmonic-reject mixer (6P-HRM) [6] can suppress the critical 3rd and 5th harmonics by around 35 dB under typical 1% gain and 1° phase errors, but it is still not the whole for a TX covering a 16×-wide RF range, i.e., harmonics up to the 15th are in-band. An active tunable-LC filter [7] can be employed to extend both harmonic rejection ratio (HRR) (42 dB) and the number of rejecting harmonics (up to 15th), but the power (171 mW) is penalized to uphold the linearity, while calibration should be adopted to surmount the LC-tank variation. Although the 18-path HRM (18P-HRM) is capable to reject harmonics up to the 15th (40 dB HRR) and suppress certain distortion sidebands, the enforced baseband (BB) input (18 paths, 18 phases) and LO format (18 phases, 33% duty cycle) are complicated. The former implies 7 more differential digital-to-analog converters and 14 more I/O pins than the 6P-HRM. The latter resorted from a div-by-9 ring counter draws substantial power (156 mW) and entails a high-frequency reference LO (LOref=9×RF), adding complexity to the frequency synthesis.
In fact, the constraint of high LOref is shared by most HRM-based architectures. For the receiver in [9], LOref=8×RF is entailed for precise 8-phase LO generation, limiting its operating bandwidth (BW) between 400 to 900 MHz (albeit a 5 GHz signal-path BW). Another example is [10], the rotational 16P-HRM entails LOref=16×RF, restricting its operating BW between 100 to 300 MHz while drawing considerable power (69.8 mW at 100 MHz).
In order to break the common constraint of high-frequency reference LO (LOref) in existing HRM-based architecture [7]-[10], this invention describes a number of techniques to realize a wideband TX with high HRR achieved on chip, measuring more favorable power (53 to 75 mW) and HRR (59.3 dB) than the prior art [7], [8].